1. Field of the Invention
The present invention relates to a satellite radionavigation receiver.
It applies in particular to the construction of a receiver of the known system of C/A code GPS satellite radionavigation, descriptions of which can be found in the publications from the Institute of Navigation, Washington D.C., entitled "Global positioning system" No. 0.936406-01-1 and No. 0.936406-00-3.
2. Discussion of the Background
In the C/A code GPS system each radionavigation receiver receives spectrally spread signals at the 1575.42 MHz frequency. These signals which are emitted by the satellites of the system, are gathered on the ground by an aerial of hemispherical radiation pattern, with a very weak signal level, of the order of -133 dBm for each satellite received. The signal received is demodulated by a receiver linked to the aerial which extracts therefrom, on the one hand, almanac and ephemeris information and, on the other hand, information about the time of propagation to the receiver of the signals emitted by the satellites and about the Doppler shift in their reception frequency. The almanac and ephemeris information is supplemented with correction coefficients which are specific to each of the satellites tracked by the receiver, and all this information is overlaid on their spread spectrum pseudo-random code. The almanac and ephemeris information enable a navigation computer to compute, at any instant, the position of the satellites tracked by the receiver. The information relating to the time of reception of the signals from the satellites and to the Doppler shift enable the navigation computer to ascertain the separation and inherent speed of the satellites tracked.
Based on the preceding information, the main tasks of the navigation computer are to compute the position of the receiver relative to the tarestrial geoid and possibly to deliver the universal time coordinated, UTC. Other cosmographical functions may also be grafted onto the main functions described earlier and in particular the functions required for navigation in specified zones of the geoid.
According to a known structure of GPS receivers, the signals from an aerial are amplified and converted into baseband analog signals by a superheterodyne receiver, generally with double frequency-conversion, owing to the large gain which must be produced. The signals obtained are possibly digitised and three processing operations are carried out on the baseband signals obtained. A first processing operation consists in performing a lead/lag correlation with a replica of the spread spectrum pseudo-random code received from the satellite in order to finely encode the distance, also called the pseudo-distance, separating the satellite from the receiver. The second processing operation consists in performing a correlation by "costas" loop with a reference clock in order to encode the rate of displacement of the satellite along the satellite/receiver radius vector, this measurement being called the "satellite delta pseudo distance" measurement. Lastly, the third processing operation consists in performing a final correlation with the reference clock in order to demodulate the DPSK almanac and ephemeris data signals and the correction coefficients overlaid on the satellite's spread spectrum code. This latter processing operation also yields a rough encoding of the satellite distance which is supplemented by the first processing operation to obtain the real pseudo distance. All these data are next processed by a navigation processor which deduces from them the position of the receiver and the time. Naturally, implementing of this structure requires the use of a significant number of components which limits, for reasons of cost and size, exploitation of the receivers to a reduced number of channels.